Circular or tubular members such as pipe and umbilicals are frequently supported by friction dogs engaging them. Most conventionally the friction dogs are in the form of slip inserts with sharp teeth which bite into the surface of the circular or tubular members to effectively give them a very high coefficient of friction. Whereas typical steel on steel friction is presumed to be in the range of 0.1, sharp teeth are calculated to have a coefficient of friction of 0.5, although as they are biting into the circular member the number is likely much higher.
If a dependable grip is desired without sharp teeth to bite into and potentially damage the part being supported, some other method of increasing the friction against the supported member needs to be done. U.S. Pat. No. 6,820,705 shows a method of doing this whereby the force against the supported member is amplified by the wedging action of a surface on the back side of the friction dog which is at a skewed angle relative to the direction of approach of the friction dog to the supported member.
As is illustrated in '705, the coefficient of friction on the back side of the slip is enhanced by the typical eight degree taper of the supporting surface. The coefficient of friction against the supported member must be enhanced more by the wedging action of that invention for the supported member to be safely supported.
The wedging action of the '705 slips requires that the friction dogs must be properly positioned before engagement for approach to the supported member. Once initially engaged with the supported member, the friction dog must then be free to move laterally along the skewed surface to effect the amplification. The support must have a combination of being fixed when, plus being flexible when needed.
Prior attempts to solve this problem have been to do the prepositioning by hand or to not control the prepositioning and simply hope it works out satisfactorily.
The need for properly, automatically, and suredly prepositioning the friction dog before engagement with the supported member and then allowing relatively free lateral movement after engagement have not been satisfactorily resolved before this invention.